Millimetre-wave detection device for discriminating between different materials

ABSTRACT

A detection device that can be used for detecting objects behind clothing etc including a dielectric lens and a receive element sensitive to millimetre wave radiation. Prior art systems produce an image of a scene usually using scanning optics. This can be large and expensive. The present invention instead take spot readings from different parts of a scene without building up an image. The spot readings are processed, and an indication given to a user if certain characteristics of the readings are observed. Typical characteristics used are the differences in absolute received power level, and the power level at different polarisations. Such characteristics are typically present is an object of interest is in the scene. Also disclosed are various methods of altering the received beam to get readings from different areas from the scene, such as changing the beam width, or beam angle.

This invention relates to a detection device. More specifically, theinvention relates to a device for detecting the presence of an objecthaving a particular property, where the background to the object, or theobject's surroundings has a differing property. The invention isparticularly aimed towards the detection of objects using millimetrewave electromagnetic radiation.

Millimetre wave systems currently exist that are able to create an imageof a scene using the radiation coming from the scene, in an analogousfashion to an ordinary camera recording a scene using radiation atvisible wavelengths. These systems produce an image of the radiometrictemperature of the scene. The frequencies used in such imagers may bebetween around 10 GHz up to around 400 GHz. Lower frequencies sufferfrom the problem of having poor resolution, whereas component costs atthe higher frequencies make the systems prohibitively expensive. If asmall system is required at reasonable cost, the reduced antenna sizewill exacerbate the problem of poor resolution, leading to much reducedperformance. For this reason, millimetre wave imagers tend to be largedevices. One such device is described in PCT publication WO 98/47020,this describing a scanning imager that has, as a preferred embodiment,an array of receive elements. Scanning optics direct the incomingmillimetre wave radiation from various portions of the target area tothese elements, and the detected radiation is processed to produce animage of the scene. The scanning is achieved using a large rotatingreflector, the rotational axis of which is inclined to the normal to theface of the disk.

Other millimetre wave imagers exist that use a single receive elementand scan radiation from various directions onto this element, to buildup an image of the scene over time. These systems generally comprise adish with a single receiver mounted at the focus. The dish is mountedsuch that it may be scanned across a scene in a raster, or othersuitable pattern. Such systems often take minutes to complete a singlescan. Reducing the quantity of receive elements in a system can resultin a cheaper system that may be designed and used much more quickly.However, each of the fewer elements will need to be scanned across awider area of coverage in order to produce an image equivalent to onecreated with more elements. This will take more time, in which the scenemay change.

It is an aim of the present invention to provide a system for thedetection of objects that is much simpler than a full imaging system,and portable and quick in operation.

According to the present invention there is provided a detection devicefor discriminating between different materials comprising an opticalsystem having at least one dielectric lens element and a receive elementcharacterised in that the receive element is sensitive tomillimetre-wave radiation, the optical system is arranged to focusincident energy from a scene onto the receive element, and the device isadapted to measure the power of a received signal at different times andfurther adapted to be able to make measurements from different parts ofthe scene, and provide an indication based on the measurements.

The present invention provides a device that does not form an image ofthe scene; it merely takes one or more measurements of the energy, orradiometric temperature, coming from a particular part or parts of thescene according to the beam properties of the antenna. This reading willvary depending upon where the antenna is pointed, and so will give theuser an idea as to the radiometric temperature of the region at whichthe antenna is pointing. The device is particularly suited to beoperable at millimetre wave frequencies, where it can be madeparticularly compact as compared to a mm-wave imaging system, and may bearranged to provide real-time readings.

The device is particularly suitable for taking radiometric measurementsof the body of a person to provide an indication as to what he or shemay be carrying, as the radiometric properties of most objects carriedwill in general be different to that of the body.

Preferably the antenna, receive element and indication means arecombined into a single, easily portable unit, such that a user canconveniently take measurements from different parts of a body to get anidea as to the variation of the radiometric temperature within thetarget area.

Preferably the antenna comprises at least one dielectric lens element.More preferably, one embodiment of the antenna comprises a plurality ofcompound lenses such that a substantially afocal telescope arrangementis formed. Preferably, the radiation emanating from the afocal telescopeis focused onto the receive element using a further dielectric lens.Alternatively the antenna may comprise of a mirror or mirrors arrangedto focus radiation from the scene onto a receive element.

The invention may also incorporate a calibration element. Thecalibration element preferably comprises a rotatable disk, the diskbeing divided into a plurality of regions, one or more beingsubstantially transparent to the received radiation, and one or morebeing opaque, where the opaque regions may comprise a materialabsorptive at the radiation frequency of interest.

Preferably, the calibration element is positioned behind the rearelement of the afocal telescope, so that it is in the narrow beam ofcollimated radiation produced by the telescope.

Whilst the invention as so far described is useful for providingdiscrimination between different parts of a target, it is generallyunable to detect radiation having a polarisation orthogonal to theorientation required by the receive element. Another embodiment enhancesthe ability of the detector to discriminate between radiation polarisedin different ways. This embodiment may incorporate polarisationsensitive elements that allow the radiometric temperature of the targetto be gauged for differing polarisations and at different parts of thetarget. Preferably, the device may provide an indication to the user ifthere is little difference in the received energy at orthogonalpolarisations whilst still detecting a difference in energy received atthe same polarisation, at different parts of the target or between thetarget and the absorptive element of the calibration disk. Thisindication may be activated if the orthogonal polarisation difference isless than a given threshold, whilst the parallel polarisation differencebetween the target and the absorptive element of the calibration disk orbetween different parts of the target is greater than a given threshold.A greater ability to detect metal objects is achieved with this scheme.The indication may be aural, visual or tactile, and may comprise ananalogue or digital meter, a sound alert, a vibration unit, or maycomprise any other suitable indication means.

The polarisation sensitive elements may be arranged to convert theincoming radiation to a single polarisation. This arrangement maycomprise of one or more quarter wave plates or Faraday rotators. Thearrangement is preferably able to provide a measure of the radiometrictemperature of an object at two orthogonal polarisations. A furtherembodiment includes at least two quarter-wave plates mounted upon arotating disk such that as the disk rotates each quarter wave plate isin turn positioned in the path of the incoming radiation. These discsmay be mounted upon the calibration element if present, such that theplates occupy the substantially transparent portions of the rotatabledisk. The plates may be arranged such that at least one pair have theirfast axes at 90° to each other. The embodiment also has a fixedquarter-wave plate mounted behind the rotating plates. There may be alinear polariser on top of some or each of the quarter-waves plate toimprove the discrimination of each polarisation.

The device may also be arranged to alter the beam pattern without a userphysically moving the antenna, such that differing readings of separate,adjacent or overlapping areas of the scene are recorded. The readingsfrom these different areas may then be compared, and an indicationprovided if the difference in readings exceeds a given threshold. Thisembodiment effectively calibrates the system by reference to thedifference between readings from different areas, and so a calibrationelement within the device would not be required.

One embodiment that achieves this has means for redirecting the beampattern of the antenna without physically moving the antenna. Preferablythis comprises means for scanning the beam in a conic fashion. This maybe done by means of a rotatable prism, which may be mounted behind theafocal telescope in the narrow collimated beam it produces.

A further embodiment incorporates means within the antenna for changingthe beamwidth, such that the spot size on the target is also changed.Radiometric temperature readings taken with different size beamwidthsmay be compared, and an indication provided if differences beyond agiven threshold are found.

An alternative embodiment that may be used for altering the beam patterndoes not use an afocal telescope, but instead employs a slab ofdielectric material rotatably mounted behind a lens, and positioned inthe path of incoming radiation focused by the lens. The slab is arrangedto have two main faces parallel to each other, with a normal to thesefaces being at a non-zero angle to the axis of rotation of the slab. Afurther embodiment may, however, incorporate both an afocal telescopeand a slab of rotatably mounted dielectric material.

According to a second aspect of the invention there is provided a methodof detecting objects present in a scene by means of receiving millimetrewave radiation from the scene, characterised in that:

-   -   a first measurement is made of radiation from a first part of        the scene;    -   a further measurement is made of radiation from a second part of        the scene;    -   an indication is provided if characteristics of the first        measurement are different to characteristics of the further        measurement.

The invention will now be described in more details, by way of exampleonly, with reference to the following figures, in which:

FIG. 1 diagrammatically illustrates a first embodiment of the currentinvention;

FIG. 2 diagrammatically illustrates the calibration element present inthe first embodiment;

FIG. 3 diagrammatically illustrates certain polarisation sensitiveelements of a second embodiment of the current invention;

FIG. 4 diagrammatically illustrates more details of a certainpolarisation sensitive element of the second embodiment;

FIG. 5 diagrammatically illustrates in more detail the design of ameanderline structure used as a polarisation sensitive element;

FIG. 6 diagrammatically illustrates further design details of ameanderline as incorporated into the second embodiment of the currentinvention;

FIG. 7 diagrammatically illustrates certain parts of a third embodimentthat incorporates means for modulating the receive beam direction;

FIG. 8 diagrammatically illustrates a typical scan pattern resultingfrom the receive beam modulation upon a scene;

FIG. 9 diagrammatically illustrates the result of changing the scancharacteristics of the third embodiment.

FIG. 10 diagrammatically illustrates a fourth embodiment thatincorporates means for modulating the receive beam width;

FIG. 11 diagrammatically illustrates the effect of modulating thebeamwidth upon a scene;

FIG. 12 diagrammatically illustrates another means for modulating thereceive beamwidth of the device;

FIG. 13 diagrammatically illustrates a fifth embodiment, thatincorporates the polarisation sensitive elements and the scanningelements present in some previous embodiments;

FIG. 14 diagrammatically illustrates the scan areas of the fifthembodiment;

FIG. 15 diagrammatically illustrates an alternative means of scanningthe beam, along with a resulting scan pattern;

FIG. 16 diagrammatically illustrates an alternative arrangement for apolarisation sensitive element;

FIG. 17 diagrammatically illustrates an alternative embodiment that doesnot employ an afocal telescope;

FIG. 18 diagrammatically illustrates alternative methods for measuringradiation at different polarisations;

FIG. 19 diagrammatically illustrates an alternative receiver system foruse with the present invention that allows improved discrimination oftarget materials; and

FIG. 20 shows a graph of the reflectivity of skin and water withfrequency.

A first embodiment of the current invention, as shown in FIG. 1, has adielectric antenna 1 formed from three dielectric lens elements 1 a, 1b, 1 c. Elements 1 a and 1 b together form an afocal telescope, suchthat a collimated beam arriving at the input 1 a is transformed to astill collimated beam at the output of element 1 b, but of narrowerdiameter. The narrower beam passes through a calibration element 2before being focused by lens element 1 c onto a receiver element 3 whichconverts the electromagnetic signal into currents passing alongelectrical wires. The receiver element 3 comprises a receive horn 3 aand amplification and/or downconversion electronics 3 b plus a detectorelement such as a diode. Due to limitations of the components used, thecurrent embodiment has a receive element 3 that is sensitive to only oneorientation of polarisation, although this is not a requirement of theinvention. At this stage the signal is amplified before being detected,and passed to circuitry able to provide an indication to the user basedupon this detected signal.

The afocal telescope 1 a, 1 b allows the diameter of the collimatedinput beam to be set, depending on the focal lengths of the elements 1 aand 1 b, such that it is convenient for other system elements such thecalibration element. The diameter of lens element 1 a is approximately150 mm, and it has a focal length of 168.7 mm, whereas element 1 b has adiameter of approximately 40 mm and a focal length of 36.5 mm. Focusingelement 1 c has a diameter of 32 mm and a focal length of 24.7 mm. Thisarrangement provides a parallel beam of width of 32 mm, at thecalibration element.

The lens elements 1 a, 1 b, 1 c are made from high density polythene,which has a dielectric constant of approximately 1.50 at the designfrequency of the equipment—this being approximately 80-100 GHz

The calibration element of this embodiment comprises a rotatable disc 2divided into four segments. This is best illustrated in FIG. 2 The disc2 is arranged in relation to the input beam such that by rotating thedisk 2 different segments are interposed in the beam's path. Twoopposing segments are filled with a radiation absorbent material 4 a, 4b which blocks the passage of radiation from the front end of theantenna, whilst the remaining two segments are clear, and freely allowthe passage of radiation from the front of the antenna through to thereceive element. The Radiation absorbent material (RAM) 4 a, 4 b acts asa “hot load” for calibration purposes; as it is at ambient temperature,it naturally emits a predictable level of radiation which is detectableby the receive element 3. This received value is used for calibrationpurposes within the detection circuitry.

The calibration disk is used to correct drift in the receive element.This drift typically occurs over a period the order of seconds or tensof seconds. Thus, a recalibration performed several times per second aswith this embodiment is enough to counter the effects of this drift, or1/f noise. The calibration effectively subtracts the measurement takenfrom the scene from the measurement taken from the hot load. Anysufficiently slow moving drift will have a negligible effect on thisresult. A motor 100 is arranged to rotate the disc 2 at a predeterminedrate. The device knows when the hot load is interrupting the beam due tothe presence of a sensor on the rotatable disk that indicates itsposition (not shown).

Note that some embodiments of the invention do not require a calibrationdisk. An alternative method that avoids the need for a calibration stepthat is suitable for certain embodiments is given below

Due to the finite beamwidth at the point where the beam passes throughthe calibration device 2, the transition time of the hot load to switchin or out of the path of the beam is also finite. During this time, thedetected energy is coming partially from the hot load, and partiallyfrom the scene. Energy received at the receive element 3 during thistransition phase is thus disregarded.

The rotation speed of the calibration disc 2 is 25 revolutions persecond, allowing for a maximum of 50 calibrations per second, and 50energy measurements per second.

A second embodiment of the current invention is illustrated in FIG. 3.The embodiment allows measurements taken to be discriminated on thebasis of the polarisation of the incoming radiation. Polarisationsensitive elements 5 have been mounted into the calibration disc,filling the space that was otherwise clear. A further polarisationsensitive element 6 has been positioned in the beam behind thecalibration/polarisation disc 2. This element 6 is fixed in orientation,and doesn't move with the rotation of the disc 2. Shown at 50 is anindication of the position of the beam in relation to the disc.

FIG. 4 is a part sectional view of the disc 2, and shows thepolarisation sensitive elements 5 in more detail. The elements 5 eachcomprise a polariser 7, behind which is mounted a quarter-wave plate 8.The polariser 7 is formed from a set of parallel conducting wires,appropriately spaced for the frequency range of interest. A typicalarrangement would be for the wires to be copper tracks laid with alinewidth of 100 microns and at a pitch (i.e. period) of 341 microns on250 micron thick TLX-9 substrate, as available from Taconic AdvancedDielectrics Division, 136 Coonbrook Road, Petersburgh, N.Y. 12158, USA.Element 5 a is identical to 5 b, except for the orientations of thepolariser 7 and the quarter wave plate 8. The polarisers 7 associatedwith each element e.g. 5 a have their directions of polarisationorthogonal to each other. Also, the fast axes of the quarter wave plates8 of each of the elements 5 are oriented orthogonal to each other.

The quarter-wave plate used in this embodiment comprises a meanderlinepolarisation twister. FIG. 5 shows the detail of the meanderlinestructure, suitable for use at 80-100 GHz. Four substrates, each havinga series of copper tracks arranged in a square-wave formation aresandwiched together, along with a blank spacer board that maintain thecorrect distance between central two active layers. The two outer activelayers 9 (Grid type A) are 15 thousandths of an inch (thou) thick,whilst the inner two active layers 10 (Grid type B) are 10 thou thick.The central spacer board 11 is 3.5 thou thick. The material used for thesubstrates 9, 10 and spacer board 11 is TLX-9, again available fromTaconic.

Fixed element 6 (shown in FIG. 4) in this embodiment is also ameanderline quarter-wave plate identical to 8 except in shape, orientedto convert the circularly polarised radiation from 5 a and 5 b back tolinear polarisation parallel to the polarisation accepted by thereceiver feed horn.

FIG. 6, along with Table 1 show the detail of the tracks that make upeach of the panels of the meanderline, with the detailed dimensions ofthe various elements of the tracks in table 1, where w1 and w2 arelinewidths, b is the periodicity, h is the height, and a is the pitch.

Necessary modifications to the design of the meanderline to account fordifferent operating frequencies will be known to those skilled in therelevant arts, and will not be discussed further herein. Further detailsrelating to meanderlines may be found in the following references: L.Young et al., IEEE Transactions on Antennas and Propagation, vol AP21,pp 376-378, May 1973, and R A Chu et al, IEEE Transactions on Antennasand Propagation, vol AP35, pp 652-661, June 1987. Details of some otherdevices that may be used in place of a meanderline for the opticalcomponent 11 are provided in The International Journal of Infrared andMillimeter Waves, Vol 2, No 3, 1981.

TABLE 1 Track dimension, microns A B H w1 w2 Grid A 294 1396 380 60 41Grid B 436 1396 592 108 141

In use, the polarisation sensitive elements 5 rotate with thecalibration disk, and hence cyclically form part of the path of thereceived beam of radiation from the scene. Care is taken to allowmeasurements of radiation from the scene to be taken only when theelements 5 are correctly positioned with respect to the receive beam. Asthe disc rotates, whilst the beam may be entirely within a particularelement 5, the rotation will cause the orientation of the polarisationelements to change. Energy measurements taken across the whole of thisregion will thus be prone to error due to the orientation of thepolarisation sensitive elements changing throughout the reading. Forthis reason, the reading taken is integrated across only 45° of rotationof the disc, when the beam occupies the central region of each element5.

The effect of the rotatable elements 5 and the fixed element 6 is toconvert horizontal polarisation (using one element e.g. 5 a) andvertical polarisation (using the orthogonal element e.g. 5 b) to thepolarisation to which the receive element is sensitive. It does this asfollows. Assume that radiation coming from the scene is plane polarised,on a horizontal axis. This radiation hitting the element having thevertical polariser will not pass through, and so will not be detected.Should the radiation instead hit the other element, it will pass throughthe polariser and be converted to circular polarisation by means of thequarter-wave plate 8 b. This circularly polarised radiation will thenpass through the fixed quarter-wave plate 6, which will convert theradiation back to linear polarisation, which is detected using the(suitably aligned) receive element.

Now assume that vertically polarised radiation is emanating from thescene. This radiation hitting the element having the horizontalpolariser will be stopped, and hence not be detected. If the radiationhits the other element 5, it will pass through to the quarter-wave plate8 a where it will again be converted to circular polarisation. However,even though the polarisation entering the quarter-wave plate 8 isorthogonal to the case described in the above paragraph, the output ofquarter-wave plate 8 a will be circular radiation having the samehandedness as described in the paragraph above, because the fast axes ofthe two rotatable quarter-wave plates 8 a and 8 b are orthogonal. Hence,when this radiation is passed through the fixed quarter-wave plate 6 itis again converted to radiation having the orientation at which thereceive element is sensitive.

In this way, a complete rotation of the disk 2 allows two readings to betaken from a scene, with each taken at differing polarisations. Thereadings taken at each polarisation can be compared, and an indicationprovided to the user based on these readings. Alternatively, tworeadings can be provided, one at each polarisation, or the two readingscan be combined to produce a composite reading of both polarisations.

A third embodiment, best illustrated in FIG. 7, allows the receive beamdirection to be modulated without physically moving the device. Thisfacility allows readings to be taken from multiple areas of a scenequickly and accurately, without the user needing to point the device tothose areas manually, and allows simple comparison of the measurementstaken at these areas. The modulation in this embodiment is carried outby means of a rotatable prism 12 mounted in front of the calibrationelement 2. As the prism 12 rotates it deflects the receive beamdirection through an angle dependent on the shape of the prism. Thepresent invention incorporates a prism 12 that has a face cut atapproximately 7.3° to the plane normal to its rotational axis. This hasthe effect of dithering the beam at the prism 12 by approximately 7.3°in total, which translates to a movement of the beam the other side ofthe afocal telescope 1 a, 1 b of approximately 1.5° in total. This issimilar to the half-power beamwidth of the receive beam, and someasurements will be taken from adjacent areas on the scene as the prism12 scans

In this embodiment the calibration element 2 may be optionallydiscarded. The system can be arranged to remove the effects of drift inthe receiver by taking as its output a difference between readingsrecorded at different parts of the scene. However, a disk is stillemployed in a similar position and is used to hold the quarter-waveplates. If the calibration elements are kept however, there is providedthe facility to differentiate between a target which is “cooler” thanits background, and a target that is “hotter” than its background, asthe calibration disk will be at a known radiometric temperature. It willbe understood by one normally skilled in the art that the words “cooler”and “hotter” refer to the radiometric temperature of the subject, ratherthan the thermal temperature.

The prism 12 is formed from high density polythene, and is rotatablymounted in path of the beam by means of bearings positioned around thecircumference of the prism 12. The prism 12 is arranged to rotate at arate one quarter that of the calibration element 2. This allows eightmeasurements to be taken per revolution of the prism, and hence the scanpattern 13 on the scene 101 will be as shown in FIG. 8.

Alternatively, the calibration disk 2 may be arranged to rotate at arate of (r+0.5) times the rate of the prism 12, where r is an integer.This will mean that an odd number of (possibly overlapping) areas on thetarget are measured, and provides the benefit that during two fullrevolutions of the prism 12, each of the measured areas on the targetare measured at both polarisations. This improves the accuracy ofmeasurements of parts of a scene taken at differing polarisations, as inthis case the areas measured during two revolutions of the prism will beexactly aligned (assuming other factors do not change). Thus the use ofpolarisation as a discriminant in deciding whether an object of interestis present is aided by using the non-integer rotation ratio. It will beclear to a person skilled in the art that other non integer ratios willalso be beneficial in this regard. It will also be clear to a personskilled in the art that using an integer relationship will allowpolarisation to be used as a discriminant, if successive measurements ofa particular area overlap, but reduced performance may result due tosuccessive measurements of a particular area not being perfectly aligned

FIG. 9 shows the effect of this alteration of rotation rate on themeasurements taken from a scene. The seven upper circles represent thecalibration disk 2 at seven different times in the measurementprocedure. The calibration disk 2 is shown divided into quadrants, withtwo opposing quadrants 203, 204 having lines representing polarisationsensitive elements arranged to pass either horizontal or verticalpolarisation respectively. The small circle 205 represents the receivedradiation passing through the calibration disk, and indicates the activepolarity at time t_(n). At each time t_(n) the disk is shown rotatedthrough half a revolution compared to t_(n-1), so that alternativepolarisations are passed, and hence measured, at successive timeintervals.

Below each of the seven representations of the calibration disk 2 aretwo further circles 200 which represent a scene being measured. Theunshaded smaller circle 201 within each circle 200 represents the pointon the scene 200 from which the measurement is being made at thatparticular time t_(n). The middle row of circles 200 represent themeasurements taken when the prism 12 is rotating at the same rate as theprism, i.e. r=1. This value of r has been chosen to illustrate theprinciple, and may not be one used in practice. It will be seen thatwhen a particular area 201 (e.g. the lower area 201 at times t=0, 2, 4,6) is being measured on the scene 200, the measurement is always made atthe same polarity.

Contrast this with the situation when r=1.5, which is represented by thelower set of seven circles 200. Successive measurements of a given area201 (again, say the lower one at times t=0, 3, 6) are now taken atalternate polarisations. The measurement of the same spot at alternatepolarisations allows better discrimination methods to be used inidentifying characteristics of any object present at that spot, asdescribed elsewhere in this specification.

A fourth embodiment of the current invention provides another means formodulating the beam, such that readings from different areas may betaken from the scene and compared to produce an output. This embodimentis shown in FIG. 10. Here, instead of changing the direction of the beamusing a rotating prism as with the previous embodiment, the beamwidth isaltered by means of changing the power of the lens 1 (shown in FIG. 1).The lens power is changed by incorporating a moveable lens element 1 dthat has the effect of changing the focal length of the lens1—effectively creating a zoom lens. The lens element 1 d may be movedlinearly along the axis of the lens 1 by means of motors 14 or othermeans. Although not shown in FIG. 10, there may be multiple lenselements 1 d that are able to move at different speeds or directions soas to improve the performance of the zoom lens.

The beam coverage on a scene a given distance from the lens 1 willtherefore change in size as the focal length of the lens is changed,creating a larger or smaller coverage “spot” on the scene. This is shownin FIG. 11. If a small but highly reflective object 15 is present in ascene 101 at which the beam is pointed when adjusted to produce a largerspot size 16, the received signal will be reasonably strong. If, whenthe lens is adjusted to produce a smaller spot size 17 the return signaldrops, it will be clear that there is an object in the region of thelarger spot 18, but not in the region of the smaller spot 17, and thatthe object 15 and its surroundings 101 have different reflectionproperties. This object 15 can then be further investigated using othermethods.

An alternative configuration for implementing the varifocal lens in thisembodiment is shown in FIG. 12. Here, instead of moving one or moreelements of the compound lens 1 axially so as to change the overallfocal length of the lens 1, different lens elements e.g. 18, each havingdifferent strengths are inserted into the compound lens arrangement 1.This has a similar effect, but is easier to implement, as the lensese.g. 18 are, in this embodiment, mounted upon rotatable disks 19. Eachdisk 19 holds four lenses 18 of differing powers, and when a measurementis being taken only one lens 18 from each disk 19 is in the path of thereceived radiation. Changing focal length of the compound lens 1 is doneby rotating the disks 19 until the correct lenses 18 are positioned inthe radiation path. The embodiment has two disks 19 each incorporatingfour individual lens elements 18. The discs 19 are preferably mountedeither side of the calibration element 2, and behind the rear afocallens element 1 b (shown in section view).

In use, one or more measurements will be taken with lenses 18 chosen soas to provide a known beamwidth. Following this, the discs 19 will berotated to select another pair of lenses that changes the beamwidth toanother known setting. One or more readings will be taken with this newsetting, and measurements taken at differing beamwidth settingscompared.

The details regarding incorporation of lens elements into an existingcompound lens arrangement so as form a zoom lens is known in the art,and so further details will not be provided herein. For more informationon the design of zoom lenses see W. J. Smith, ‘Modern lens design—aresource manual’, Ch. 16.3 pp 292-299, McGraw-Hill 1992

A fifth embodiment is shown in FIG. 13. This embodiment incorporates thepolarisation sensitive elements 5, 6 discussed in the second embodiment,along with the beam scanning elements 12 discussed in the thirdembodiment. The prism is arranged to rotate at a quarter of the rate ofthe calibration element. Thus, one revolution of the prism will resultin eight measurements being taken, each at alternate polarisations.

The embodiment of FIG. 13 can be employed to give a greater degree ofdiscrimination towards metallic objects in a scene. To do this, readingsfrom two or more areas of the scene are taken at orthogonalpolarisations. If the readings taken at the same polarisation fromdifferent parts of the scene produce differing power returns, this isindicative of the presence of an object on the target. Next, thereadings at orthogonal polarisations are examined. If these readings aresimilar to each other, this will indicate that the object is likely tobe metallic in nature, due to the reflectance properties of metals atnon-glancing angles. In a similar fashion, the system can be arranged tobe more sensitive to non-metallic objects by looking for a suitablylarge difference in the readings taken at orthogonal polarisations. Suchindications are used to trigger an alert to an operator indicating thatfurther investigation may be required.

The scan pattern of the fifth embodiment is shown in FIG. 14. The areawithin each circle represents most of the energy that is received in anindividual measurement, and also the approximate shape of the half powerbeam upon a scene. Here, it can be seen that alternate scans are takenat alternate polarisations, due to the action of the rotating polarisingelements. The circular half power beam shape shown is only approximatebecause in practice the rotation of the system elements during eachmeasurement will result in the individual scan areas being slightlyellipsoid in shape, although this has no disadvantage in practice.

FIG. 15 a shows an alternative means for scanning the beam, this timebetween two discrete positions as opposed to the effectively conicalscanning arrangement discussed in relation to the third embodiment.Scanning prism 300 replaces the rotating prism 12 shown in FIG. 7. Thisprism 300 comprises two segments 301, 302, and in use rotates about axis303 such that each segment 301, 302 is in the path of the incomingradiation for approximately 50% of the time. The segments 301, 302 areshaped such that for the duration of a segment being fully in theradiation path, it directs the incoming radiation 303 from a fixedangular region in space to the receive element (not shown). Each segmentis arranged to direct radiation from a different region. Each segment301, 302 is shaped like a portion of the side of a cone having an axis303, but the segments 301, 302 are inverses of each other to enableradiation from differing regions to be focused onto the receive element.

FIG. 15 b shows the resulting scan pattern on a scene comprising a body304. Two regions 305, 306 are shown from where the radiation receivedwill be focused onto the receive element, each corresponding to adifferent segment 301, 302 of the scanning prism 300. The scanningarrangement shown is more efficient than previous embodiments asradiation from the fixed region is viewed for more of the angularrotation time as compared with previous embodiments. More energy can bereceived in this greater time, leading to improved system signal tonoise ratios.

FIG. 16 shows an alternative arrangement for the disk 2 as shown in FIG.3. As the operation of the polarisation sensitive elements 5 of FIG. 3is dependent upon their angular position in relation to the fixedpolarisation element 6, there is only a relatively short angular windowin which valid measurements can be made, leading to reduced signalintegration times and hence reduced system signal to noise ratios. Theembodiment of FIG. 16 increases this angular window, by incorporatingdisk 307 having a revised arrangement of polarisation sensitive elementse.g 308. The alternative disk 307, as shown in FIG. 16 has a greaternumber of segments e.g. 308 occupying the same angular space as theprevious disk 2. Each of the segments e.g. 308 therefore covers asmaller angular range, and so a greater portion of each segment can bearranged to be within the angular window in which valid measurements canbe made, as opposed to having a larger segment where only the centralportion is within that angular window as discussed in relation to FIGS.4, 5 and 6.

The disk 307 has alternate quarter-wave plate segments e.g. 308arranged, in combination with fixed element 6 (shown in FIG. 3) toconvert horizontal and vertical polarisations respectively to the stateto which the receive element 3 (shown in FIG. 1) is sensitive.

To reduce the required disk diameter, the disk 307 may be mountedbetween lens 1 c and receive element 3 (as shown in FIG. 1), where thebeamwidth is smaller, allowing the area of the polarisation sensitiveelements to be reduced.

When employed together, the revised scanning means discussed in relationto FIG. 15 and the revised arrangement of polarisation sensitiveelements discussed in relation to FIG. 16 improve the energy collectionefficiency of the system from 2% for the embodiment shown in FIG. 7 upto approximately 20%.

In a further embodiment both the prism 300 in FIG. 15 a and thepolarisation sensitive elements disc 307 in FIG. 16 are mounted on thesame shaft and thus rotate at the same speed, reducing mechanicalcomplexity. A scan efficiency of approximately 20% is achievable. Inanother implementation the polarisation sensitive elements disc may havean odd number of sectors (with one extra sector of a particularpolarisation received compared to the other polarisation) and the prismperforms a half-turn every time the polarisation sensitive elements discmoves the beam from the centre of one sector to the centre of theadjacent sector. In this implementation a scan efficiency ofapproximately 30% is achievable but at the cost of mechanicalcomplexity—the prism and disc have separate shafts and drives, and theprism has to spin very rapidly leading to potential problems withbalancing, aerodynamic drag and power requirements.

FIG. 17 shows an alternative embodiment that does not employ an afocaltelescope and which is able to scan a beam. This may make the opticalarrangement more compact, leading to a smaller detector device. Lens 20is made from polythene, and is mounted in front of parallel faced slab21. The slab 21 is mounted such that it can rotate about an axis 23,being driven by a motor (not shown). A normal to one of the parallelfaces 22 is arranged at an angle to the axis 22. For instance, if thefront lens 1 a of the afocal telescope used in the previous embodimentsis used on its own, a parallel sided slab of high density polythene 16.9mm thick rotating about the optical axis of the system and tilted at 20degrees to the optical axis of the system would perform the equivalentscanning function to the prism in the previous embodiments. This slabwould be of a size so that it is just contained in a cylinder ofdiameter 28.2 mm centred on the optical axis, and the rear surface ofthe slab at the intersection with the optical axis would be 12.9 mm infront of the focal plane. A receive element 24 is mounted at the focalpoint of the system, as in previous embodiments. The lens 20 is arrangedto direct radiation from a scene onto a parallel face 22 of the slab 21.The position of slab 21 at any given instant will govern where in thescene the radiation comes from that is finally focused onto the receiveelement 24, due to refraction effects within the slab 21. Radiationcoming from an upper part of the scene passing through the lens 20 willbe directed towards the lower half of the region between the lens 20 andthe slab 21. When the slab in is position 21 it will tend to directradiation from this lower half onto the receive element 24. Conversely,radiation coming from a lower part of the scene will be directed more into the upper half of the region between lens 20 and the slab 21. Thiswill tend to be focused onto the receive element 24 when the slab 21 hasmoved into the position 21′. Thus, by rotating the slab 21 the receivebeam is directed in a conical scan.

The polarisation dependent elements described in other embodiments maybe included in the embodiment shown in FIG. 17. However the normallyskilled person will realise that in this embodiment the beam is alwaysconverging, and so the size of any polarisation dependent elements willneed to be determined appropriately according to their position withinthe system.

FIG. 18 shows two alternative methods for measuring a scene at differingpolarisations. These both rely on the polarisation dependence of thereceive element. FIG. 18 a shows one-approach where the receiver module25, including the feedhorn 26 and receive element (not shown) are allrotated through 90° between measurements, about the shaft 28 using amotor (not shown), thus allowing the two orthogonal polarisations to bemeasured. Slip rings may be used for the transfer of power and signallines 27 to the receiver module 25.

FIG. 18 b shows an alternative technique wherein the feed horn 26 isarranged rotate on its own, being connected to the receiver module 30 bya waveguide rotary joint 29. This would give more repeatable resultscompared to the embodiment of FIG. 18 a since there would be norequirement for slip rings, however a waveguide rotary joint 29, awell-known component to any person skilled in the art (e.g. G. C.Southworth, ‘Principles and applications of waveguide transmission’, pp364-366, D. Van Nostrand Company Inc, 1950), comprises twolinear-to-circular waveguide converters 31, 32 as well as the joint 29itself, so is heavy and of limited bandwidth—which would reduce thesensitivity of the device.

FIG. 19 shows a means for improving the discrimination of materials upona body, and hence may more reliably indicate the presence of asuspicious object. The Figure shows a revised receiving means (broadlyequivalent to that of item 3 of FIG. 1) comprising a receive horn 309,which supplies received energy to amplification electronics 310. Theoutput of the amplifier 310 is split into two paths 311, 312. Path 311goes via band pass filter 313 and then on to detector means 314. Path312 goes via band pass filter 315 and then on to detector means 316.Filter 313 is arranged to pass most strongly those signals having afrequency at the lower end of the amplifier or waveguide bandwidth forexample between 75 GHz and 80 GHz, whereas filter 315 is arranged topass most strongly those signals having a frequency at the higher end ofthe amplifier or waveguide bandwidth for example between 105 GHz and 110GHz. This allows received energy at each passband to be separatelydetected, and hence compared.

As shown in FIG. 20, the reflection from skin at normal incidence isdifferent at each frequency band. The solid trace 317, representing thereflectance of skin is seen to largely mirror the reflectance of water,indicated by the dashed trace 318. This is not surprising, as skin ismade up largely from water. In the lower frequency band, indicated byline 319, the reflectivity is seen to be approximately 0.375, whereasthis drops to approximately 0.34 for the higher band, indicated by line320.

Incorporating the receiving means of FIG. 19 therefore allows ameasurement taken at a given instant to be examined to see whether thisdifference in received power at each frequency band is present in themeasurement. If it is not, then this is an indication that themeasurement does not come from skin, and so is more likely to be eithermetallic or some other substance. This information may be fed in to anydecision as to whether to trigger an alert to an operator, and may beused to reduce false alarm rates.

The skilled person will be aware that other embodiments within the scopeof the invention may be envisaged, and thus the invention should not belimited to the embodiments as herein described. In particular, featuressuch as the polarisation dependent elements, and different scanningmeans disclosed herein may clearly be interchangeable betweenembodiments and their appearance on a given embodiment does not meanthat they cannot be utilised on other embodiments.

1. A detection device for discriminating between different materialscomprising an optical system having at least one antenna element and areceive element characterised in that the receive element is sensitiveto millimetre-wave radiation, the optical system is arranged to focusincident energy from a scene onto the receive element, and the device isadapted to measure the power of a received signal at different times andfurther adapted to include a beam steerer for redirecting an incomingdirection of the incident energy to be able to make measurements fromdifferent parts of the scene, and provide an indication based on themeasurements without using said measurements in the formation of animage of the scene.
 2. A detection device as claimed in claim 1 whereinthe device is adapted to measure radiation at a plurality ofpolarisations from the scene.
 3. A detection device as claimed in claim2 wherein means for altering the polarisation of the radiation withinthe device is incorporated in the optical system.
 4. A detection deviceas claimed in claim 3 wherein the receive element is sensitive to afirst polarisation state, and the means for altering the polarisationperiodically alters the polarisation of radiation orthogonal to thefirst polarisation state such that it is in the first polarisationstate.
 5. A detection device as claimed in claim 4 wherein the polaritychanging means incorporates a fixed quarter-wave plate and at least onemoveable quarter-wave plate arranged such that the position of the (atleast one) moveable quarter-wave plate determines which polarisation ofthe radiation incident upon the optical system will be detectable by thereceive element.
 6. A detection device as claimed in claim 5 wherein thequarter-wave plates are fitted with polarising elements.
 7. A detectiondevice as claimed in claim 5 wherein the at least one moveablequarter-wave plate is rotatably mounted such that radiation incidentupon the optical system may be directed through the at least onemoveable quarter-wave plate, and at different angular positions theradiation passing through the at least one quarter wave plate seesorthogonal fast axes.
 8. A detection device as claimed in claim 5wherein the quarter-wave plates comprise meanderline structures.
 9. Adetection device as claimed in claim 1 wherein the device includes aninternal millimetre-wave source arranged to periodically provide areference signal to the receive element, the reference signal being usedin calibration of the device.
 10. A detection device as claimed in claim9 wherein the internal millimetre-wave source comprises a radiationabsorbent material rotatably mounted such that it periodicallyinterrupts the path of the radiation received by the optical system. 11.A detection device as claimed in claims 1 wherein the device is arrangedto change the direction of arrival of the incoming radiation with time.12. A detection device as claimed in claim 11 wherein the device isarranged to make successive measurements at orthogonal polarisations.13. A detection device as claimed in claim 12 wherein the device isarranged to measure successive measurements in a particular direction atorthogonal polarisations.
 14. A detection device as claimed in claim 11wherein a refractive element is mounted in the path of the receivedradiation, the refractive element being rotatable such that differentrotational positions result in energy from differing directions beingpassed to the receive element.
 15. A detection device as claimed inclaim 14 wherein the refractive element comprises a prism.
 16. Adetection device as claimed in claim 14 wherein the refractive elementcomprises at least one segment of a cone.
 17. A detection device asclaimed in claim 14 wherein the refractive element comprises a parallelfaced slab rotatably mounted such that a normal to a parallel face doesnot lie on the axis of the antenna.
 18. A detection device as claimed inclaim 1 wherein the device is arranged to change the beamwidth of areceive beam with time.
 19. A detection device as claimed in claim 18wherein the beamwidth is arranged to be changed by means of changing thefocal length of one or more lens elements making up the optical system.20. A detection device as claimed in claim 19 wherein the means forchanging the focal length of one or more of the lens elements comprisesapparatus for switching different lenses into the path of the receivedradiation.
 21. A detection device as claimed in claim 1 wherein theoptical system comprises an afocal telescope.
 22. A method of detectingobjects present in a scene by means of receiving millimetre waveradiation from the scene using a scanning optical system, wherein: afirst measurement is made of radiation from a first scanned part of thescene; a further measurement is made of radiation from a second scannedpart of the scene wherein the second part of the scene may overlap withthe first part of the scene; and an indication is provided ifcharacteristics of the first measurement are different tocharacteristics of the further measurement without using saidmeasurements to form an image of the scene.
 23. A method as claimed inclaim 22 wherein an observed characteristic is the received power level.24. A method as claimed in claim 23 wherein power levels at orthogonalpolarisations are used as an observed characteristic.
 25. A method asclaimed in claim 22 wherein the incoming radiation is focused onto areceive element by means of an optical system.
 26. A method as claimedin claim 25 wherein the optical system incorporates scanning means tochange with time the direction of arrival of the incoming radiation suchthat measurements from different parts of the scene are taken.
 27. Amethod as claimed in claim 25 wherein the receive element is sensitiveto the polarisation of the incoming radiation, and means is incorporatedfor altering the polarisation of incoming radiation.
 28. A detectiondevice for discriminating between different materials comprising anoptical system having at least one antenna element and a receive elementcharacterized in that the receive element is sensitive tomillimeter-wave radiation, the optical system is arranged to focusincident energy from a scene onto the receive element, and the device isadapted to measure the power of a received signal at different times andfurther adapted to include optics for changing the beamwidth of theantenna to allow measurements from different sized parts of the scene,and provide an indication based on the measurements, without using saidmeasurements in the formation of an image of the scene.